Count all substrings having character K

Given a string str and a character K, the task is to find the count of all the substrings of str that contain the character K.

Examples:

Input: str = “geeks”, K = ‘g’
Output: 5
“g”, “ge”, “gee”, “geek” and “geeks” are the valid substrings.



Input: str = “geeksforgeeks”, K = ‘k’
Output: 56

Naive approach A simple approach will be to find all the substrings having character K of the given string and return the count;

Efficient approach: For every index i in the string, find the first index j such that i ≤ j and str[j] = K. Now, the substrings str[i…j], str[i…j + 1], str[i…j + 2], …, str[i…n – 1] will all contain the character K at least once. The approach seems to be O(n2) at first but the index j will not be calculated again for every index i, j will be a valid index for all the values of i less than j.

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the index of the
// next occurrence of character ch in str
// starting from the given index
int nextOccurrence(string str, int n,
                   int start, char ch)
{
    for (int i = start; i < n; i++) {
  
        // Return the index of the first
        // occurrence of ch
        if (str[i] == ch)
            return i;
    }
  
    // No occurrence found
    return -1;
}
  
// Function to return the count of all
// the substrings of str which contain
// the character ch at least one
int countSubStr(string str, int n, char ch)
{
  
    // To store the count of valid substrings
    int cnt = 0;
  
    // Index of the first occurrence of ch in str
    int j = nextOccurrence(str, n, 0, ch);
    for (int i = 0; i < n; i++) {
        while (j != -1 && j < i) {
            j = nextOccurrence(str, n, j + 1, ch);
        }
  
        // No occurrence of ch after index i in str
        if (j == -1)
            break;
  
        // Substrings starting at index i
        // and ending at indices j, j+1, ..., n-1
        // are all valid substring
        cnt += (n - j);
    }
  
    return cnt;
}
  
// Driver code
int main()
{
  
    string str = "geeksforgeeks";
    int n = str.length();
    char ch = 'k';
  
    cout << countSubStr(str, n, ch);
  
    return 0;
}

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Java

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// Java implementation of the approach
import java.util.*;
  
class GFG
{
      
    // Function to return the index of the 
    // next occurrence of character ch in str 
    // starting from the given index 
    static int nextOccurrence(String str, int n, 
                              int start, char ch) 
    
        for (int i = start; i < n; i++) 
        
      
            // Return the index of the first 
            // occurrence of ch 
            if (str.charAt(i) == ch) 
                return i; 
        
      
        // No occurrence found 
        return -1
    
      
    // Function to return the count of all 
    // the substrings of str which contain 
    // the character ch at least one 
    static int countSubStr(String str, 
                           int n, char ch) 
    
      
        // To store the count of valid substrings 
        int cnt = 0
      
        // Index of the first occurrence of ch in str 
        int j = nextOccurrence(str, n, 0, ch); 
        for (int i = 0; i < n; i++) 
        
            while (j != -1 && j < i)
            
                j = nextOccurrence(str, n, j + 1, ch); 
            
      
            // No occurrence of ch after index i in str 
            if (j == -1
                break
      
            // Substrings starting at index i 
            // and ending at indices j, j+1, ..., n-1 
            // are all valid substring 
            cnt += (n - j); 
        
        return cnt; 
    
      
    // Driver code 
    static public void main ( String []arg)
    
        String str = "geeksforgeeks"
        int n = str.length(); 
        char ch = 'k'
          
        System.out.println(countSubStr(str, n, ch)); 
    
}
  
// This code is contributed by PrinciRaj1992

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Python3

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# Python3 implementation of the approach
  
# Function to return the index of the
# next occurrence of character ch in strr
# starting from the given index
def nextOccurrence(strr, n, start, ch):
    for i in range(start, n):
  
        # Return the index of the first
        # occurrence of ch
        if (strr[i] == ch):
            return i
  
    # No occurrence found
    return -1
  
# Function to return the count of all
# the substrrings of strr which contain
# the character ch at least one
def countSubStr(strr, n, ch):
  
    # To store the count of valid substrrings
    cnt = 0
  
    # Index of the first occurrence of ch in strr
    j = nextOccurrence(strr, n, 0, ch)
  
    for i in range(n):
        while (j != -1 and j < i):
            j = nextOccurrence(strr, n, j + 1, ch)
  
        # No occurrence of ch after index i in strr
        if (j == -1):
            break
  
        # Substrrings starting at index i
        # and ending at indices j, j+1, ..., n-1
        # are all valid substrring
        cnt += (n - j)
  
    return cnt
  
# Driver code
strr = "geeksforgeeks"
n = len(strr)
ch = 'k'
  
print(countSubStr(strr, n, ch))
  
# This code is contributed by Mohit Kumar

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C#

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// C# implementation of the approach 
using System;
  
class GFG
{
      
    // Function to return the index of the 
    // next occurrence of character ch in str 
    // starting from the given index 
    static int nextOccurrence(String str, int n, 
                               int start, char ch) 
    
        for (int i = start; i < n; i++) 
        
      
            // Return the index of the first 
            // occurrence of ch 
            if (str[i] == ch) 
                return i; 
        
      
        // No occurrence found 
        return -1; 
    
      
    // Function to return the count of all 
    // the substrings of str which contain 
    // the character ch at least one 
    static int countSubStr(String str, 
                           int n, char ch) 
    
      
        // To store the count of valid substrings 
        int cnt = 0; 
      
        // Index of the first occurrence of ch in str 
        int j = nextOccurrence(str, n, 0, ch); 
        for (int i = 0; i < n; i++) 
        
            while (j != -1 && j < i)
            
                j = nextOccurrence(str, n, j + 1, ch); 
            
      
            // No occurrence of ch after index i in str 
            if (j == -1) 
                break
      
            // Substrings starting at index i 
            // and ending at indices j, j+1, ..., n-1 
            // are all valid substring 
            cnt += (n - j); 
        
        return cnt; 
    
      
    // Driver code 
    static public void Main () 
    
        String str = "geeksforgeeks"
        int n = str.Length; 
        char ch = 'k'
          
        Console.WriteLine(countSubStr(str, n, ch)); 
    
}
  
// This code is contributed by AnkitRai01

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Output:

56



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